To find side $ a $ use formula:
$$ R = \dfrac{ \sqrt{ 2 } \cdot a }{ 2 } $$After substituting $R = 6\, \text{cm}$ we have:
$$ 6\, \text{cm} = \dfrac{ \sqrt{ 2 } \cdot a }{ 2 } $$ $$ \sqrt{ 2 } \cdot a = 6\, \text{cm} \cdot 2 $$ $$ \sqrt{ 2 } \cdot a = 12\, \text{cm} $$ $$ a = \dfrac{ 12\, \text{cm} }{ \sqrt{ 2 } } $$ $$ a = 6 \sqrt{ 2 }\, \text{cm} $$